Breast cancer is the most common female cancer and approximately 70-75% of cases express estrogen receptor alpha (ER?). Tamoxifen (TAM) is an estrogen receptor antagonist and the standard endocrine therapy for premenopausal women with ER? positive breast cancer. Unfortunately, resistance to endocrine therapy develops in almost all advanced tumors. Prolylcarboxypeptidase (PRCP) and its family member prolylendopeptidase (PREP) are members of the prolyl- peptidase family. These enzymes cleave neuropeptide G-protein coupled receptor (GPCR) agonists to regulate GPCR signaling. PRCP was identified in a genetic screen for factors that promote TAM resistance in MCF7 cells. Preliminary data show high PRCP expression is correlated with worse prognosis in breast cancer patients. PRCP over-expression increased AKT-mTOR activation, promoted TAM resistance, and induced spontaneous metastasis in MCF7 tumor xenografts. We identified a potent inhibitor of PRCP and PREP termed Y-ox. PRCP/PREP depletion or inhibition by Y-ox destabilized IRS-1 and inhibited AKT-mTOR. Because PRCP and PREP cleave peptide GPCR agonists, we tested if PRCP/PREP regulate IRS-1 and the AKT-mTOR pathway in a GPCR-dependent manner. Our data support a model in which PRCP/PREP increase GPCR-dependent activation of CaMK2 (calcium/calmodulin activated kinase 2), and activated CaMK2 then stabilizes IRS-1 by inhibiting AMPK. Finally, we showed Y-ox destabilized IRS-1 and inhibited feedback activation of AKT in rapamycin treated cells and, in combination with rapamycin increased killing of TAM- resistant cells. Based on these results, we hypothesize 1) PRCP/PREP maintain IRS-1 and the AKT/mTOR pathway in a GPCR and CaMK2-dependent manner, leading to TAM resistance and metastasis, 2) PRCP, PREP, and/or CaMK2 expression in primary breast tumors will correlate with TAM resistance and poor prognosis, 3) combined inhibition of PRCP/PREP and mTOR will reduce feedback activation of the PI3K/AKT and consequently improve treatment of endocrine resistant and metastatic breast tumors.